The present invention relates to an improved process for halogen bleaching of pulp to obtain a significant increase in the brightness of the pulp without the need to increase the amount of halogen compounds used during the pulp bleaching process and without significant loss of pulp properties such as viscosity and yield.
The process of bleaching pulp for use in paper applications may be performed with halogen containing or non-halogen containing bleaching agents. Due to public perception and environmental concerns over chlorinated organics and dioxins, it has become necessary to reduce the use of halogen containing bleaching agents. An attempt to reduce the use of halogen compounds during the bleaching process by use of peroxide and/or non-halogen containing oxygen compounds often results in a detrimental effect on the characteristics of the pulp including decreased brightness, viscosity and yield.
One reason for the decrease in brightness of the pulp is the presence of metal ions in the bleached pulp. In an attempt to remove metals from the pulp, chelating agents have been added during the peroxide bleaching stage (P) of a non-halogen bleaching process and/or subsequent to the last alkaline stage since many of the metals are introduced into the pulp with the water used in the extraction and wash stages of the bleaching process. None of these prior uses of chelating agents has been found to be effective for decreasing the amount of halogenated compounds required to bleach the pulp in a halogen and non-halogen bleaching process while maintaining the target brightness, viscosity and yield characteristics of the pulp.
It is an object of the present invention to provide a process which reduces the amount of chlorinated organics in paper products and effluent from a paper bleaching process.
It is another object of the invention to provide a halogen bleaching process wherein the efficiency of the bleaching process is improved without adversely affecting the pulp viscosity or yield.
A further object of the invention is to provide a method for reducing the use of chlorinated compounds in a pulp bleaching process while obtaining a target brightness for the bleached pulp.
Having regard to the above and other objects, the present invention provides a method for bleaching a pulp containing lignocellulosic fibers with one or more halogen containing compounds, preferably chlorine and/or chlorine dioxide, whereby the amount of adsorbable organic halide content of the bleached pulp is significantly reduced. The method comprises contacting the pulp during one or more chlorine bleaching stages with an amount of chelating agent sufficient to increase the brightness of the pulp over the brightness of pulp which has been treated with a chelating agent after a final washing or extraction stage of the bleaching process, whereby an adsorbable organic halide content of the bleached pulp is significantly reduced.
It has been discovered, quite surprisingly, that contacting the pulp with one or more chelating agents during at least one of the halogen bleaching, wash or extraction sequences provides a pulp having superior brightness as compared to pulp bleached with the same amount of halogen compounds in the absence of the chelating agent. Furthermore, an increase in the pulp viscosity may be obtained by adding one or more oxygen stabilizers during a subsequent non-chlorine bleaching stage. Accordingly, the foregoing benefits may be obtained without significantly increasing the amount of organic halide compounds in the pulp or effluent from the bleaching process. These results are truly remarkable since they do not require substantial changes in the bleaching process and thus may be implemented without significantly increasing the complexity or operating cost of the bleaching process and without the need for significant capital expenditures.
In another aspect the invention provides a process for bleaching a pulp containing lignocellulosic fibers. The process comprises maintaining the pulp at a consistency in the range of from about 0.5% to about 40% and maintaining the pH in the range of from about 1 to about 9. The pulp is then bleached with a chlorine compound in a bleaching stage. During the bleaching or before a washing stage, the pulp is contacted with from about 0.01 wt. % to about 1 wt. % chelating agent. The amount of chelating agent used is based on the dry weight of pulp. This process has been found to significantly reduce the amount of adsorbable organic halide in the bleached pulp.
In a preferred embodiment, the invention provides a process for bleaching a pulp containing lignocellulosic fibers with chlorine and non-chlorine containing bleaching agents, wherein the pulp has a consistency in the range of from about 0.5% to about 40% and has a pH in the range of from about 1 to about 9. The process comprises treating the pulp with chlorine dioxide in a first chlorination stage. Subsequent to the first chlorination stage, the pulp is contacted with a first amount of metal chelating agent, wherein the amount of chelating agent is sufficient to reduce an organic halide content of the fibers and wherein the temperature of the pulp during the contacting is within the range of from about 35xc2x0 to about 110xc2x0 C. Next the chlorine bleached pulp is bleached with a chlorine-free bleaching agent after a subsequent extraction stage resulting in bleached pulp having a low level of chlorinated organic compounds.